Typical examples of hair dye compositions include two-part permanent hair dyes composed of a first part containing an alkali agent, an oxidation dye and optionally, a direct dye such as nitro dye and a second part containing an oxidizing agent; and one-part semi-permanent hair dyes containing an organic acid or an alkali agent, and at least one direct dye such as acid dye, basic dye or nitro dye.
Permanent hair dyes however have the drawback that their color tone produced by an oxidation dye is not so vivid. It is the common practice to try to produce vivid color by using a direct dye and an oxidation dye in combination. Direct dyes, particularly nitro dyes, however have the problem that color of the hair dyed with them fades considerably and very vivid color tone of the hair just after dyeing is lost rapidly.
Direct dyes usable in combination with oxidation dyes are limited because they are required to be stable against peroxides during the hair dyeing process.
Various acid dyes, basic dyes, and nitro dyes to be added to permanent dyes or semi-permanent dyes are apt to fade in any case because direct dyes on the hair are lost relatively easily by shampooing or exposure to light. Such a tendency is marked in the damaged hair.
For improving resistance (fastness) to light, shampooing, sweat, friction, or heat, a hair dye using a direct dye having a dissociative proton is proposed (refer to Patent Documents 1 and 2). In hair dyes using direct dyes, however, a good balance in dyeing property or fading property between the root and tip of the hair tends to be achieved by mixing a plurality of dyes having the same hue (refer to Non-Patent Document 1) so that there is a demand for the development of dissociative azo dyes different in the structural characteristics.    [Patent Document 1] JP-A-2004-107343    [Patent Document 2] JP-A-2006-182653    [Non-Patent Document 1] Clarence R. Robbins, Chemical and Physical Behavior of Human Hair, 4th Ed., pp 331-334, Springer-Verlag, 2002